Cullen / Cassandra
Hm. That doesn't narrow the suspect pool considerably, if we're including everybody who might have talked with someone from Thedas. Most of the people here from Thedas can be ruled out, though - only Krem, Bethany, and Varric are unattached. Since Bethany's attention has apparently been elsewhere, that leaves Krem and Varric.
They specifically referenced embrium and royal elfroot. Of the lot of us who are here, the Warden-Commander is the only potions master, and therefore the only one likely to be mentioning an herb as rare as royal elfroot. It's certainly not something that comes up in my day to day conversations. Unfortunately, the Warden-Commander is probably also the one who talks to the most people, given her work here.